The 2009 H1N1 influenza pandemic exemplified the threat of animal influenza viruses emerging in a human population that lacks cross-protective immunity. There were notable observations in 2009-10 that immune mediators were a factor in severe pandemic H1N1 (panH1N1) cases and that an inactivated seasonal trivalent vaccine in some cohorts enhanced the risk of severe panH1N1 illness. A similar phenomenon of enhanced disease is observed in pigs after vaccination with an inactivated H1 swine influenza virus, followed by heterologous panH1N1 challenge. Therefore, our long-term goal is to elucidate the pivotal factors that determine if influenza vaccines cross-protect or exacerbate disease upon infection with antigenically divergent strains. Our objectives in this proposal are to identify which compartment of the immune response and which viral constituents in an inactivated vaccine are integral to disease enhancement in a vaccinated host infected with heterologous panH1N1. We will meet the objectives by an experimental approach that includes the following specific aims: (1) Determine by passive immunization if antibodies elicited by an inactivated H1 vaccine predispose pigs for enhanced disease when infected with the heterologous panH1N1 virus. (2) Using recombinant vaccines, determine if mismatch between viral envelope proteins of the vaccine strain and panH1N1 is sufficient to cause enhanced disease following panH1N1 challenge. The rationale behind the proposed study is that it will answer vital questions, in a natural influenza host, about the mechanisms by which an antigenically mismatched vaccine can enhance infection with a strain novel to the human population. The results will also focus future mechanistic studies into the critical viral antigens (e.g. specific proteins and epitopes targeted) and host immune factors (e.g. complement-fixing IgG leading to granulocyte infiltration; CTL-mediated epithelial cell death). These investigations will be significant because they will provide new criteria for improved seasonal vaccine design and delivery, especially for contingencies when novel strains adapt to circulate in humans. Achieving the stated aims and progressing toward our long-term research goal will also deliver significant insight into a novel mechanism that disrupts a normal response to viral infection of the respiratory tract. Probing our questions about severe, immune- related panH1N1 disease in an animal model outside conventional platforms of human disease research is an innovative approach to understanding a difficult public health problem.